Messages in this thread | | | Date | Wed, 13 Feb 2019 17:20:47 +0000 | From | Will Deacon <> | Subject | Re: [RFC PATCH] docs/memory-barriers.txt: Rewrite "KERNEL I/O BARRIER EFFECTS" section |
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[+Tony]
On Mon, Feb 11, 2019 at 02:34:31PM -0800, Linus Torvalds wrote: > On Mon, Feb 11, 2019 at 9:30 AM Will Deacon <will.deacon@arm.com> wrote: > > > > + > > + 1. All readX() and writeX() accesses to the same peripheral are ordered > > + with respect to each other. For example, this ensures that MMIO register > > + writes by the CPU to a particular device will arrive in program order. > > Hmm. I'd like more people look at strengthening this one wrt across > CPUs and locking. > > Right now we document mmiowb(), but that "documentation" is really > just a fairy tale. Very *very* few drivers actually do mmiowb() on > their own. > > IOW, we should seriously just consider making the rule be that locking > will order mmio too. Because that's practically the rule anyway.
I would /love/ to get rid of mmiowb() because I think it's both extremely difficult to use and also pretty much never needed. It reminds me a lot of smp_read_barrier_depends(), which we finally pushed into READ_ONCE for Alpha.
> Powerpc already does it. IO within a locked region will serialize with the > lock.
I thought ia64 was the hold out here? Did they actually have machines that needed this in practice? If so, I think we can either:
(a) Add an mmiowb() to their spin_unlock() code, or (b) Remove ia64 altogether if nobody complains
I know that Peter has been in favour of (b) for a while...
Will
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